1. Field of the Invention
The present invention relates to a method for producing a laminated porous polyolefin film which is superior in mechanical strength, air permeability and interlayer adhesiveness and which is free of pinholes.
2. Description of the Related Art
Porous films made of polyolefin resin have been employed in various fields such as clothes, filters and cell separators. Especially, porous films made of resin having a high strength, namely, resin having a low MI, are employed because a high mechanical strength is generally required in the use of porous films as cell separators. A development of porous films meeting the demand for further improvement in performance such as strength and ion permeability with recent increase in cell capacity is going on.
As a method for producing a laminated porous film made of a common polyolefin resin, for example, a method comprising drawing a multilayer extrusion formed laminated film to form a porous film is disclosed in Japanese Patent Laid-Open No. 62-79806. However, this method has a problem that occurrence of foreign substance contamination or fish eyes during the formation of a laminated film will result in appearance of pinholes penetrating the film through the processing of the laminated film into a porous film. On the other hand, polyolefin resin which can be molded by multilayer extrusion generally has an MI of about 2 to 100. Laminated porous films made of such polyolefin resin problematically have poor strength.
In light of such situations, as one example of a method for obtaining a laminated porous film made of a low-MI resin, namely, an ultra high molecular weight resin, a method comprising thermally bonding a single layer porous film made of an ultra high molecular weight polyethylene and a porous film resulting from a 1.3-fold drawing of the foregoing porous is disclosed in Japanese Patent Laid-Open No. 6-182918.
However, this method has a problem that favorable air permeability and favorable interlayer adhesiveness cannot be achieved simultaneously because porous films are laminated together. In other words, lamination conducted at a low temperature such that pores are not closed will result problematically in an insufficient adhesion strength which will cause delamination of layers. On the other hand, lamination conducted at a high temperature so that a sufficient adhesion strength is achieved will problematically cause pores to close, resulting in an insufficient air permeability.